Electroluminescent light-sources (LEDs) emit light in a narrow region of the spectrum. What are termed phosphor-converted LEDs (pcLEDs) convert at least part of the primary radiation emitted by the LED into secondary radiation. This process is called light conversion and the materials used for the purpose are called light-converting materials. A pcLED can thus emit mixed light covering a wider region of the spectrum (wideband emission) as a result of the mixing of primary and secondary radiation. For the light conversion, the LED is coated with, for example, a light-converting layer of a powdered light-converting phosphor material that is embedded in an organic matrix. This arrangement constitutes a simple light-source for emitting wideband light (mixed light produced by the mixing of primary and secondary radiation), and particularly white light. Because of the operating temperatures of LEDs, the optical properties of the light-converting layer change as the time in operation becomes longer, particularly as a result of the organic matrix becoming colored, which means that intensity is not stable over time and there is a shift in the color point of the mixed light.
Document WO2006/072918 A1 describes a phosphor-converted arrangement having a plurality of identically colored blue LEDs, for emitting white light that is produced by mixing blue primary radiation and phosphor-converted secondary radiation. To produce the secondary radiation, use was made of a ceramic light-converting material whose optical properties, and in particular its transmission and its converting capacity, are not sensitive to the effects of temperature during the operation of the LEDs. Given the same light yield per LED, the brightness of the arrangement was increased by using two LEDs. As well as a first light-converting material to produce yellow secondary radiation, the ceramic light-converting layer also comprises in this case a further material for producing red secondary radiation, whereby wideband white light having a high color rendering index is emitted. However, the disadvantages of an arrangement of this kind are the color point of the mixed light, which varies with the angle of viewing, the energy losses that occur in the course of the conversion of the blue primary radiation into red secondary radiation, and the resulting low efficiency of this pcLED arrangement.
It is therefore an object of the present invention to provide an arrangement for the efficient emission of mixed light, which mixed light has a color point that is stable and not dependent on the angle of viewing.